264 PRINCIPLES OF STRATIGRAPHY 



stricted to glacial denudation. /. e.. the removal and transport of 

 weathered material. (See ante, Chapter I.) 



The first step in glacial erosion is generally ablation, or the 

 taking up of the weathered material on the surface of the land. 

 When ice accumulates through a long period of gradual refrigera- 

 tion, punctuated by intervals of partial melting, the waterlogged 

 soil, the product of pre-glacial weathering, is incorporated as an in- 

 tegral part into the base of the glacier or ice sheet and carried 

 away with it when movement begins. Through shearing and 

 through formation of ascending ice currents this material may be 

 carried upward, and, from being subglacial, becomes successively 

 englacial and then superglacial. The part remaining in the bottom 

 of the ice is used as a tool for the corrasion of the rock surface, 

 which soon is worn down to the undecomposed layers and becomes 

 smoothed, polished, and marked by parallel striae, the direction of 

 which indicate the direction of ice movement. The material held 

 in the base of the ice sheet (the product of denudation plus material 

 which may have descended from the surface) is likewise polished 

 and striated, this in the Hatter fragments of rock usually being con- 

 fined to two faces. 



The heat generated by friction over the rock surfaces, aug- 

 mented by the rising heat of the earth, owing to the ice blanketing, 

 may result in melting the basal portion of the ice sheet, or glacier, 

 whereupon a layer of basally transported material or till is deposited 

 on the previously eroded surface, protecting it from further erosion. 

 If the direction of motion of the ice is subsequently changed, it may 

 not again remove this material, but leave it to protect the striae 

 made during the early period of movement (Crosby). That ice is 

 able to move over unconsolidated material without actively eroding it 

 is shown by many observations (Fairchild-22). Glacial grooves of 

 exceptional size are sometimes formed, the best known being those 

 of Kelley's Island in Lake Erie. The most effective work of cor- 

 rasion by glaciers seems to occur somewhere above the lower end of 

 the glacier, so that deepening of the valley above its mouth results. 

 Both glaciers and ice sheets effect little or no erosion at their front 

 or margins. At tiie head of the glacier, i. e., at the Bergschrund, 

 plucking or pulling out of loose blocks occurs, resulting in the 

 formation of cirques. Plucking also occurs on the lee side of pro- 

 jecting rock masses which on their stoss side are eroded by corra- 

 sion, the result being a rock mass with smooth upper surface, grad- 

 ually rising in the direction of ice movement, and terminating in a 

 rough cliff downstream. These structures are known as roches mou- 



